[0001] The embodiments described herein generally relate to a system and method for improved
processing of received data depending on channel conditions in a communication system.
[0002] In voice and data communications networks, there is an on-going need to minimize
bandwidth requirements and improve the quality of voice or data traffic. Reducing
the bandwidth is typically achieved by implementing compression algorithms to remove
redundancy from a signal. On the other hand, signal quality is typically improved
by adding redundancy to a signal by, for example, implementing error detection and
correction techniques, and by recovering from errors by using lost frame concealment
techniques.
[0003] Conventional systems attempt to achieve a balance between bandwidth and quality by
using a combination of methods. Generally, in a conventional system, at the transmitting
side, a source coder/quantizer is provided to quantize and compress the signal to
be transmitted, i.e. reduce the bandwidth required, while a channel coder is provided
to add information for use in error detection and correction, i.e. improve quality.
The signal then travels through a channel (data link) where it may be corrupted. At
the receiving side, a corresponding channel decoder, lost frame handler and source
decoder are provided to decode the signal received.
[0004] One of the issues in communication systems is that, as the interference level increases,
the quality of recovered signal falls off rapidly. One conventional approach to overcome
this problem has been the use of adaptive source/channel coding (e.g. GSM's Adaptive
Multi-Rate (AMR)). Adaptive source/channel coding allows a variation in the level
of source coding based on the amount of interference found on the channel data link.
For example, a lower level of source coding is performed when the level of interference
is high. This allows for more redundancy in the signal and thus, the interference
will have less impact on the signal. However, this also has the effect of increasing
bandwidth requirements. In a similar way, when the level of interference is low, a
higher level of source coding can be used. In this way, adjustments can be made adaptively
to counteract the effects of interference during signal transmission.
[0005] While adaptive source/channel coding adjusts the source coder based on interference
conditions, other conventional approaches are directed to the receiver side of the
channel. In a communication system, when a data bit is received, there is some uncertainty
as to whether or not the bit is a 1 or a 0 due to distortion, interference, noise
on the channel, or the like. In a conventional system, the channel decoder would typically
examine an incoming signal and make a decision to determine whether a particular received
bit is a 1 or a 0.
[0006] A source decoder then receives the bits and processes this data using various well-known
techniques depending on the processing performed by the channel decoder to output
a recovered signal. However, prior to processing by the source decoder, lost frame
concealment techniques are employed to deal with frames of data that are lost or otherwise
damaged.
[0007] [0006a]
EP-A1 624 585 discloses a transmission system with a transmitter having a channel encoder and a
receiver having a channel decoder. The channel encoder derives encoded symbols from
source symbols and the channel decoder reconstructs the source symbols from a signal
containing the encoded symbols. The signal has been transmitted over a transmission
channel from the transmitter to the receiver. The transmitter also includes a separate
encoder for coding a coding property that is then transmitted to the receiver. The
coding property is used by the encoder. The receiver includes a separate decoder for
decoding the encoded coding property. The decoded coding property is then used to
set a coding property of the channel decoder.
General
[0008] In one aspect, at least one embodiment described herein may provide a lost frame
concealment method for processing data frames received from transmission over a communications
channel wherein the method comprises: determining whether a current data frame is
a bad frame or a good frame, a data frame being a bad frame when determined to be
received with error or used for control purposes and the data frame being a good frame
when determined to be received without error and not used for control purposes; performing
source decoding on the current data frame with one or more parameters; wherein if
the current data frame is a bad frame the one or more parameters are limited by a
first set of one or more values; and if the current data frame is a good frame and
a previous data frame is a bad frame a quality of the communications channel is checked
to determine whether to limit the one or more parameters.
[0009] The method may comprise performing source decoding on the current data frame with
one or more parameters, wherein the one or more parameters are not limited when the
current and previous data frames are good frames.
[0010] Alternatively, if the current data frame is a good frame and the previous data frame
is a bad data frame, the method may further comprise determining a value for a channel
quality indicator to determine the quality of the communications channel by comparing
the value of the channel quality indicator to a threshold; performing the step of
source decoding on the current data frame with one or more parameters, wherein the
one or more parameters are not limited if the quality of the communications channel
is good; and performing source decoding on the current data frame with one or more
parameters, the one or more parameters being limited by a second set of one or more
values if the condition of the communications channel is bad.
[0011] The second set of parameters can be different from the first set of parameters.
[0012] The channel quality indicator can be one of a Bit Error Rate (BER), a BLock Error
Ratio (BLER), a Signal to Noise Ratio (SNR) and a specially defined parameter that
indicates channel condition.
[0013] In at least some instances, the data frames include speech frames, and the method
may be applied in an Adaptive Multi-Rate (AMR) speech decoding for concealing the
effect of lost AMR speech frames.
[0014] In at least some instances, a state machine may be used to indicate the quality of
the communications channel, and the method further comprises: starting the state machine
in state 0; incrementing a state counter to enter a subsequent numbered state each
time a bad frame is detected, the incrementing being limited to 6; and resetting the
state counter to zero each time a good speech frame is detected except when in state
6 in which case the state counter is set to 5.
[0015] In these instances, the step of_source decoding on the current data frame with one
or more parameters wherein the one or more parameters are not limited is performed
in state 0, in which the method comprises not limiting LTP gain and fixed codebook
gain, performing normal source decoding and saving the current frame of speech parameters.
[0016] Also in these instances, the steps of mentioned in paragraph 9 above may be performed
in state 0 or state 5 when the current data frame is a good data frame and the previous
data frame is a bad data frame, and the step od performing source decoding on the
current data frame with one or more parameters, the one or more parameters being limited
by a second set of one or more values if the condition of the communications channel
is bad, comprises limiting LTP gain and fixed codebook gain below values used for
the last subframe in the last received good speech frame according to:
where
gp is a current LTP gain that is applied to the current speech frame,
gp(-1) is the LTP gain that was used for the last subframe in the last good received
speech frame,
gc is a current decoded fixed codebook gain that is applied to the current speech frame
and
gc(-1) is a fixed codebook gain that was used for the last subframe of the last good
received speech frame, and the method further comprises using any remaining received
speech parameters normally, and saving the speech parameters for the current speech
frame.
[0017] Also in these instances, the step of performing source decoding on the current data
frame with one or more parameters, wherein the one or more parameters are not limited
may be performed in all states when the current data frame is a bad data frame, and
said step comprises limiting LTP gain and fixed codebook gain below values used for
the last subframe in the last received good speech frame according to
and
where
gp is a current decoded LTP gain,
gp(-1),... ,
gp(-
n) are LTP gains used for the last n subframes,
median5() is a 5-point median operation,
P(state) is
an attenuation factor defined by: (
P(1) = 0.98,
P(2) = 0.98,
P(3) = 0.8,
P(4) = 0.3,
P(5) = 0.2,
P(6) = 0.2),
gc is a current decoded fixed codebook gain,
gc(-1),...,
gc(-
n) are fixed codebook gains used for the last n subframes,
C(state) is an attenuation factor defined by: (
C(1) = 0.98,
C(2) = 0.98,
C(3) = 0.98,
C(4) = 0.98,
C(5) = 0.98,
C(6) = 0.7), state is the state value, and n is a positive integer.
[0018] In another aspect, at least one exemplary embodiment described herein may provide
a computer program product comprising a computer readable medium embodying program
code means executable by a processor of a communications device for implementing the
lost frame concealment method for processing data frames received from transmission
over a communications channel.
[0019] In another aspect, at least one exemplary embodiment described herein may provide
a communications device comprising a microprocessor configured to control the operation
of the communications device; a communication subsystem connected to the microprocessor,
the communication subsystem being configured to send and receive wireless data over
a communications channel; a channel decoder configured to decode data frames received
over the communications channel; and a lost frame handler configured to process the
received data frames for lost frames, the lost frame handler being configured to perform
the steps of the method described above.
[0020] In another aspect, at least one exemplary embodiment described herein may provide
a communication system for coding and decoding an information signal sent through
a communications channel comprising an encoder configured to encode the information
signal and send the encoded information signal over the communications channel; and
a decoder configured to receive and decode the encoded information signal to produce
a recovered signal, wherein the decoder is configured to perform the steps of the
method described above.
Brief Description of the Drawings
[0021] For a better understanding of the following embodiments described herein, and to
show more clearly how the various embodiments described herein may be carried into
effect, reference will be made, by way of example, to the accompanying drawings which
show at least one exemplary embodiment, and in which:
FIG. 1 is a block diagram of a mobile device in one exemplary implementation;
FIG. 2 is a block diagram of an exemplary embodiment of a communication subsystem
component of the mobile device of FIG. 1;
FIG. 3 is a block diagram of a node of a wireless network in one exemplary implementation;
FIG. 4 is a block diagram illustrating components of a host system in one exemplary
implementation;
FIG. 5 illustrates an exemplary embodiment of a system for coding and decoding a signal
in a channel;
FIG. 6 illustrates a flow chart diagram of an exemplary embodiment of a lost frame
concealment method;
FIG. 7 illustrates a state diagram that can be used in an exemplary lost frame concealment
method; and
FIG. 8 illustrates a flow chart diagram of another exemplary embodiment of a lost
frame concealment method.
Description of Preferred Embodiments
[0022] It will be appreciated that for simplicity and clarity of illustration,
where considered appropriate, reference numerals may be repeated among the figures
to indicate corresponding or analogous elements. In addition, numerous specific details
are set forth in order to provide a thorough understanding of the embodiments described
herein. However, it will be understood by those of ordinary skill in the art that
the embodiments described herein may be practiced without these specific details.
In other instances, well-known methods, procedures and components have not been described
in detail so as not to obscure the embodiments described herein. Also, the description
is not to be considered as limiting the scope of the embodiments described herein.
The detailed description begins with a general description of a mobile environment
and then proceeds to describe the application of exemplary embodiments within this
environment.
[0023] The mobile environment involves use of a mobile device. A mobile device is a two-way
communication device with advanced data communication capabilities having the capability
to communicate with other computer systems, and is also referred to herein generally
as a mobile device. The mobile device may also include the capability for voice communications.
Depending on the functionality provided by a mobile device, it may be referred to
as a data messaging device, a two-way pager, a cellular telephone with data messaging
capabilities, a wireless Internet appliance, or a data communication device (with
or without telephony capabilities). A mobile device communicates with other devices
through a network of transceiver stations. To aid the reader in understanding the
structure of a mobile device and how it communicates with other devices, reference
is made to FIGS. 1 through 3.
[0024] Referring first to FIG. 1, a block diagram of a mobile device in one example implementation
is shown generally as 100. Mobile device 100 comprises a number of components, the
controlling component being microprocessor 102. Microprocessor 102 controls the overall
operation of mobile device 100. Communication functions, including data and voice
communications, are performed through communication subsystem 104. Communication subsystem
104 receives messages from and sends messages to a wireless network 200. In this exemplary
implementation of mobile device 100, communication subsystem 104 is configured in
accordance with the Global System for Mobile Communication (GSM) and General Packet
Radio Services (GPRS) standards. The GSM/GPRS wireless network is used worldwide and
it is expected that these standards will be superseded eventually by Enhanced Data
GSM Environment (EDGE) and Universal Mobile Telecommunications Service (UMTS). New
standards are still being defined, but it is believed that they will have similarities
to the network behavior described herein, and it will also be understood by persons
skilled in the art that embodiments are intended to use any other suitable standards
that are developed in the future. The wireless link connecting communication subsystem
104 with network 200 represents one or more different Radio Frequency (RF) channels,
operating according to defined protocols specified for GSM/GPRS communications. With
newer network protocols, these channels are capable of supporting both circuit switched
voice communications and packet switched data communications.
[0025] Although the wireless network associated with mobile device 100 is a GSM/GPRS wireless
network in one exemplary implementation of mobile device 100, other wireless networks
may also be associated with mobile device 100 in variant implementations. Different
types of wireless networks that may be employed include, for example, data-centric
wireless networks, voice-centric wireless networks, and dual-mode networks that can
support both voice and data communications over the same physical base stations. Combined
dual-mode networks include, but are not limited to, Code Division Multiple Access
(CDMA) or CDMA2000 networks, GSM/GPRS networks (as mentioned above), and future third-generation
(3G) networks like EDGE and UMTS. Some older examples of data-centric networks include
the Mobitex
™ Radio Network and the DataTAC
™ Radio Network. Examples of older voice-centric data networks include Personal Communication
Systems (PCS) networks like GSM and Time Division Multiple Access (TDMA) systems.
Other network communication technologies that may be employed include, for example,
Integrated Digital Enhanced Network (iDEN
™), Evolution-Data Optimized (EV-DO), High Speed Downlink Packet Access (HSDPA), and
Wireless LAN technology as specified in the 802.11 series of standards.
[0026] Microprocessor 102 also interacts with additional subsystems such as a Random Access
Memory (RAM) 106, flash memory 108, display 110, auxiliary input/output (I/O) subsystem
112, data port 114, keyboard 116, speaker 118, microphone 120, short-range communications
subsystem 122 and other devices 124.
[0027] Some of the subsystems of mobile device 100 perform communication-related functions,
whereas other subsystems may provide "resident" or on-device functions. By way of
example, display 110 and keyboard 116 may be used for both communication-related functions,
such as entering a text message for transmission over network 200, and device-resident
functions such as a calculator or task list. Operating system software used by microprocessor
102 is typically stored in a persistent store such as flash memory 108, which may
alternatively be a read-only memory (ROM) or similar storage element (not shown).
Those skilled in the art will appreciate that the operating system, specific device
applications, or parts thereof, may be temporarily loaded into a volatile store such
as RAM 106.
[0028] Mobile device 100 may send and receive communication signals over network 200 after
required network registration or activation procedures have been completed. Network
access is associated with a subscriber or user of a mobile device 100. To identify
a subscriber, mobile device 100 may require a Subscriber Identity Module or "SIM"
card 126 to be inserted in a SIM interface 128 in order to communicate with a network.
SIM 126 is one type of a conventional "smart card" used to identify a subscriber of
mobile device 100 and to personalize the mobile device 100, among other things. Without
SIM 126, mobile device 100 is not fully operational for communication with network
200. By inserting SIM 126 into SIM interface 128, a subscriber can access all subscribed
services. Services could include: web browsing and messaging such as e-mail, voice
mail, Short Message Service (SMS), and Multimedia Messaging Services (MMS). More advanced
services may include: point of sale, field service and sales force automation. SIM
126 includes a processor and memory for storing information. Once SIM 126 is inserted
in SIM interface 128, it is coupled to microprocessor 102. In order to identify the
subscriber, SIM 126 contains some user parameters such as an International Mobile
Subscriber Identity (IMSI). An advantage of using SIM 126 is that a subscriber is
not necessarily bound by any single physical mobile device. SIM 126 may store additional
subscriber information for a mobile device as well, including datebook (or calendar)
information and recent call information.
[0029] Mobile device 100 includes a battery interface 132 for receiving one or more batteries
130. The battery 130 may be rechargeable. Battery interface 132 is coupled to a regulator
(not shown), which assists battery 130 in providing power V+ to mobile device 100.
Although current technology makes use of a battery, future technologies such as micro
fuel cells may provide the power to mobile device 100.
[0030] Microprocessor 102, in addition to its operating system functions, enables execution
of software applications on mobile device 100. A set of applications that control
basic device operations, including data and voice communication applications, will
normally be installed on mobile device 100 during its manufacture. Another application
that may be loaded onto mobile device 100 would be a personal information manager
(PIM). A PIM has functionality to organize and manage data items of interest to a
subscriber, such as, but not limited to, e-mail, calendar events, voice mails, appointments,
and task items. A PIM application has the ability to send and receive data items via
wireless network 200. PIM data items may be seamlessly integrated, synchronized, and
updated via wireless network 200 with the mobile device subscriber's corresponding
data items stored and/or associated with a host computer system. This functionality
creates a mirrored host computer on mobile device 100 with respect to such items.
This can be particularly advantageous where the host computer system is the mobile
device subscriber's office computer system.
[0031] Additional applications may also be loaded onto mobile device 100 through network
200, auxiliary I/O subsystem 112, data port 114, short-range communications subsystem
122, or any other suitable subsystem 124. This flexibility in application installation
increases the functionality of mobile device 100 and may provide enhanced on-device
functions, communication-related functions, or both. For example, secure communication
applications may enable electronic commerce functions and other such financial transactions
to be performed using mobile device 100.
[0032] Data port 114 enables a subscriber to set preferences through an external device
or software application and extends the capabilities of mobile device 100 by providing
for information or software downloads to mobile device 100 other than through a wireless
communication network. The alternate download path may, for example, be used to load
an encryption key onto mobile device 100 through a direct and thus reliable and trusted
connection to provide secure device communication. Data port 114 may be a suitable
port that enables data communication between the mobile device 100 and another computing
device. For example, the data port 114 may be a serial or parallel port and may also
include a power line to provide power to the mobile device 100, as is available with
Universal Serial Bus (USB) ports.
[0033] Short-range communications subsystem 122 provides for communication between mobile
device 100 and different systems or devices, without the use of network 200. For example,
subsystem 122 may include an infrared device and associated circuits and components
for short-range communication. Examples of short range communication would include
standards developed by the Infrared Data Association (IrDA), Bluetooth, and the 802.11
family of standards developed by IEEE.
[0034] In use, a received signal such as a text message, an e-mail message, or web page
download will be processed by communication subsystem 104 and input to microprocessor
102. Microprocessor 102 will then process the received signal for output to display
110 or alternatively to auxiliary I/O subsystem 112. A subscriber may also compose
data items, such as e-mail messages, for example, using keyboard 116 in conjunction
with display 110 and possibly auxiliary I/O subsystem 112. Auxiliary subsystem 112
may include devices such as: a touch screen, mouse, track ball, infrared fingerprint
detector, or a roller wheel with dynamic button pressing capability. Keyboard 116
is an alphanumeric keyboard and/or telephone-type keypad. A composed item may be transmitted
over network 200 through communication subsystem 104.
[0035] For voice communications, the overall operation of mobile device 100 is substantially
similar, except that the received signals would be output to speaker 118, and signals
for transmission would be generated by microphone 120. Alternative voice or audio
I/O subsystems, such as a voice message recording subsystem, may also be implemented
on mobile device 100. Although voice or audio signal output is accomplished primarily
through speaker 118, display 110 may also be used to provide additional information
such as the identity of a calling party, duration of a voice call, or other voice
call related information.
[0036] Referring now to FIG. 2, a block diagram of the communication subsystem component
104 of FIG. 1 is shown. Communication subsystem 104 comprises a receiver 150, a transmitter
152, one or more embedded or internal antenna elements 154, 156, Local Oscillators
(LOs) 158, and a processing module such as a Digital Signal Processor (DSP) 160.
[0037] The particular design of communication subsystem 104 is dependent upon the network
200 in which mobile device 100 is intended to operate, thus it should be understood
that the design illustrated in FIG. 2 serves only as one example. Signals received
by antenna 154 through network 200 are input to receiver 150, which may perform such
common receiver functions as signal amplification, frequency down conversion, filtering,
channel selection, and analog-to-digital (A/D) conversion. A/D conversion of a received
signal allows more complex communication functions such as demodulation and decoding
to be performed in DSP 160. In a similar manner, signals to be transmitted are processed,
including modulation and encoding, by DSP 160. These DSP-processed signals are input
to transmitter 152 for digital-to-analog (D/A) conversion, frequency up conversion,
filtering, amplification and transmission over network 200 via antenna 156. DSP 160
not only processes communication signals, but also provides for receiver and transmitter
control. For example, the gains applied to communication signals in receiver 150 and
transmitter 152 may be adaptively controlled through automatic gain control algorithms
implemented in DSP 160.
[0038] The wireless link between mobile device 100 and a network 200 may contain one or
more different channels, typically different RF channels, and associated protocols
used between mobile device 100 and network 200. An RF channel is a limited resource
that must be conserved, typically due to limits in overall bandwidth and limited battery
power of mobile device 100.
[0039] When mobile device 100 is fully operational, transmitter 152 is typically keyed or
turned on only when it is sending to network 200 and is otherwise turned off to conserve
resources. Similarly, receiver 150 is periodically turned off to conserve power until
it is needed to receive signals or information (if at all) during designated time
periods.
[0040] Referring now to FIG. 3, a block diagram of a node of a wireless network is shown
as 202. In practice, network 200 comprises one or more nodes 202. Mobile device 100
communicates with a node 202 within wireless network 200. In the exemplary implementation
of FIG. 3, node 202 is configured in accordance with General Packet Radio Service
(GPRS) and Global Systems for Mobile (GSM) technologies. Node 202 includes a base
station controller (BSC) 204 with an associated tower station 206, a Packet Control
Unit (PCU) 208 added for GPRS support in GSM, a Mobile Switching Center (MSC) 210,
a Home Location Register (HLR) 212, a Visitor Location Registry (VLR) 214, a Serving
GPRS Support Node (SGSN) 216, a Gateway GPRS Support Node (GGSN) 218, and a Dynamic
Host Configuration Protocol (DHCP) 220. This list of components is not meant to be
an exhaustive list of the components of every node 202 within a GSM/GPRS network,
but rather a list of components that are commonly used in communications through network
200.
[0041] In a GSM network, MSC 210 is coupled to BSC 204 and to a landline network, such as
a Public Switched Telephone Network (PSTN) 222 to satisfy circuit switched requirements.
The connection through PCU 208, SGSN 216 and GGSN 218 to the public or private network
(Internet) 224 (also referred to herein generally as a shared network infrastructure)
represents the data path for GPRS capable mobile devices. In a GSM network extended
with GPRS capabilities, BSC 204 also contains a Packet Control Unit (PCU) 208 that
connects to SGSN 216 to control segmentation, radio channel allocation and to satisfy
packet switched requirements. To track mobile device location and availability for
both circuit switched and packet switched management, HLR 212 is shared between MSC
210 and SGSN 216. Access to VLR 214 is controlled by MSC 210.
[0042] Station 206 is a fixed transceiver station. Station 206 and BSC 204 together form
the fixed transceiver equipment. The fixed transceiver equipment provides wireless
network coverage for a particular coverage area commonly referred to as a "cell".
The fixed transceiver equipment transmits communication signals to and receives communication
signals from mobile devices within its cell via station 206. The fixed transceiver
equipment normally performs such functions as modulation and possibly encoding and/or
encryption of signals to be transmitted to the mobile device in accordance with particular,
usually predetermined, communication protocols and parameters, under control of its
controller. The fixed transceiver equipment similarly demodulates and possibly decodes
and decrypts, if necessary, any communication signals received from mobile device
100 within its cell. Communication protocols and parameters may vary between different
nodes. For example, one node may employ a different modulation scheme and operate
at different frequencies than other nodes.
[0043] For all mobile devices 100 registered with a specific network, permanent configuration
data such as a user profile is stored in HLR 212. HLR 212 also contains location information
for each registered mobile device and can be queried to determine the current location
of a mobile device. MSC 210 is responsible for a group of location areas and stores
the data of the mobile devices currently in its area of responsibility in VLR 214.
Further VLR 214 also contains information on mobile devices that are visiting other
networks. The information in VLR 214 includes part of the permanent mobile device
data transmitted from HLR 212 to VLR 214 for faster access. By moving additional information
from a remote HLR 212 node to VLR 214, the amount of traffic between these nodes can
be reduced so that voice and data services can be provided with faster response times
and at the same time requiring less use of computing resources.
[0044] SGSN 216 and GGSN 218 are elements added for GPRS support; namely packet switched
data support, within GSM. SGSN 216 and MSC 210 have similar responsibilities within
wireless network 200 by keeping track of the location of each mobile device 100. SGSN
216 also performs security functions and access control for data traffic on network
200. GGSN 218 provides internetworking connections with external packet switched networks
and connects to one or more SGSN's 216 via an Internet Protocol (IP) backbone network
operated within the network 200. During normal operations, a given mobile device 100
must perform a "GPRS Attach" to acquire an IP address and to access data services.
This requirement is not present in circuit switched voice channels as Integrated Services
Digital Network (ISDN) addresses are used for routing incoming and outgoing calls.
Currently, all GPRS capable networks use private, dynamically assigned IP addresses,
thus requiring a DHCP server 220 connected to the GGSN 218. There are many mechanisms
for dynamic IP assignment, including using a combination of a Remote Authentication
Dial-In User Service (RADIUS) server and DHCP server. Once the GPRS Attach is complete,
a logical connection is established from a mobile device 100, through PCU 208, and
SGSN 216 to an Access Point Node (APN) within GGSN 218. The APN represents a logical
end of an IP tunnel that can either access direct Internet compatible services or
private network connections. The APN also represents a security mechanism for network
200, insofar as each mobile device 100 must be assigned to one or more APNs and mobile
devices 100 cannot exchange data without first performing a GPRS Attach to an APN
that it has been authorized to use. The APN may be considered to be similar to an
Internet domain name such as "myconnection.wireless.com".
[0045] Once the GPRS Attach is complete, a tunnel is created and all traffic is exchanged
within standard IP packets using any protocol that can be supported in IP packets.
This includes tunneling methods such as IP over IP as in the case with some IPSecurity
(IPsec) connections used with Virtual Private Networks (VPN). These tunnels are also
referred to as Packet Data Protocol (PDP) Contexts and there are a limited number
of these available in the network 200. To maximize use of the PDP Contexts, network
200 will run an idle timer for each PDP Context to determine if there is a lack of
activity. When a mobile device 100 is not using its PDP Context, the PDP Context can
be deallocated and the IP address returned to the IP address pool managed by DHCP
server 220.
[0046] Referring now to FIG. 4, a block diagram illustrating components of a host system
in one exemplary configuration is shown. Host system 250 will typically be a corporate
office or other local area network (LAN), but may instead be a home office computer
or some other private system, for example, in variant implementations. In this example
shown in FIG. 4, host system 250 is depicted as a LAN of an organization to which
a user of mobile device 100 belongs.
[0047] LAN 250 comprises a number of network components connected to each other by LAN connections
260. For instance, a user's desktop computer 262a with an accompanying cradle 264
for the user's mobile device 100 is situated on LAN 250. Cradle 264 for mobile device
100 may be coupled to computer 262a by a serial or a Universal Serial Bus (USB) connection,
for example. Other user computers 262b are also situated on LAN 250, and each may
or may not be equipped with an accompanying cradle 264 for a mobile device. Cradle
264 facilitates the loading of information (e.g. PIM data, private symmetric encryption
keys to facilitate secure communications between mobile device 100 and LAN 250) from
user computer 262a to mobile device 100, for example, through data port 114, and may
be particularly useful for bulk information updates often performed in initializing
mobile device 100 for use. The information downloaded to mobile device 100 may include
certificates used in the exchange of messages. It will be understood by persons skilled
in the art that the cradle 264 is not required to connect the mobile device 100 to
the computer 262a and that computers 262a, 262b can also be connected to other peripheral
devices not explicitly shown in FIG. 4.
[0048] Furthermore, only a subset of network components of LAN 250 are shown in FIG. 4 for
ease of exposition, and it will be understood by persons skilled in the art that LAN
250 will generally comprise additional components not explicitly shown in FIG. 4,
for this exemplary configuration. More generally, LAN 250 may represent a smaller
part of a larger network (not shown) of the organization, and may comprise different
components and/or be arranged in different topologies than that shown in the example
of FIG. 4.
[0049] In this example, mobile device 100 communicates with LAN 250 through a node 202 of
wireless network 200 and a shared network infrastructure 224 such as a service provider
network or the public Internet. Access to LAN 250 may be provided through one or more
routers (not shown), and computing devices of LAN 250 may operate from behind a firewall
or proxy server 266.
[0050] In a variant implementation, LAN 250 comprises a wireless VPN router (not shown)
to facilitate data exchange between the LAN 250 and mobile device 100. The concept
of a wireless VPN router is new in the wireless industry and implies that a VPN connection
can be established directly through a specific wireless network to mobile device 100.
The possibility of using a wireless VPN router has only recently been available and
could be used when Internet Protocol (IP) Version 6 (IPV6) arrives into IP-based wireless
networks. This new protocol will provide enough IP addresses to dedicate an IP address
to every mobile device, making it possible to push information to a mobile device
at any time. An advantage of using a wireless VPN router is that it could be an off-the-shelf
VPN component, not requiring a separate wireless gateway and separate wireless infrastructure
to be used. A VPN connection can be a Transmission Control Protocol (TCP)/IP or User
Datagram Protocol (UDP)/IP connection to deliver the messages directly to mobile device
100 in this variant implementation.
[0051] Messages intended for a user of mobile device 100 are initially received by a message
server 268 of LAN 250. Such messages may originate from any of a number of sources.
For instance, a message may have been sent by a sender from a computer 262b within
LAN 250, from a different mobile device (not shown) connected to wireless network
200 or to a different wireless network, or from a different computing device or other
device capable of sending messages, via the shared network infrastructure 224, and
possibly through an application service provider (ASP) or Internet service provider
(ISP), for example.
[0052] Message server 268 typically acts as the primary interface for the exchange of messages,
particularly e-mail messages, within the organization and over the shared network
infrastructure 224. Each user in the organization that has been set up to send and
receive messages is typically associated with a user account managed by message server
268. One example of a message server 268 is a Microsoft Exchange
™ Server. In some implementations, LAN 250 may comprise multiple message servers 268.
Message server 268 may also be adapted to provide additional functions beyond message
management, including the management of data associated with calendars and task lists,
for example.
[0053] When messages are received by message server 268, they are typically stored in a
message store (not explicitly shown), from which messages can be subsequently retrieved
and delivered to users. For instance, an e-mail client application operating on a
user's computer 262a may request the e-mail messages associated with that user's account
stored on message server 268. These messages are then typically be retrieved from
message server 268 and stored locally on computer 262a.
[0054] When operating mobile device 100, the user may wish to have e-mail messages retrieved
for delivery to the handheld. An e-mail client application operating on mobile device
100 may also request messages associated with the user's account from message server
268. The e-mail client may be configured, either by the user or by an administrator,
possibly in accordance with an organization's information technology (IT) policy,
to make this request at the direction of the user, at some pre-defined time interval,
or upon the occurrence of some pre-defined event. In some implementations, mobile
device 100 is assigned its own e-mail address, and messages addressed specifically
to mobile device 100 are automatically redirected to mobile device 100 as they are
received by message server 268.
[0055] To facilitate the wireless communication of messages and message-related data between
mobile device 100 and components of LAN 250, a number of wireless communications support
components 270 may be provided. In this example implementation, wireless communications
support components 270 comprise a message management server 272, for example. Message
management server 272 is used to specifically provide support for the management of
messages, such as e-mail messages, that are to be handled by mobile devices. Generally,
while messages are still stored on message server 268, message management server 272
can be used to control when, if, and how messages should be sent to mobile device
100. Message management server 272 also facilitates the handling of messages composed
on mobile device 100, which are sent to message server 268 for subsequent delivery.
[0056] For example, message management server 272 may: 1) monitor the user's "mailbox" (e.g.
the message store associated with the user's account on message server 268) for new
e-mail messages; 2) apply user-definable filters to new messages to determine if and
how the messages will be relayed to the user's mobile device 100; 3) compress and
encrypt new messages (e.g. using an encryption technique such as Data Encryption Standard
(DES), Triple DES or Advanced Encryption Standard (AES)) and 4) push them to mobile
device 100 via the shared network infrastructure 224 and wireless network 200; and
receive messages composed on mobile device 100 (e.g. encrypted using Triple DES),
decrypt and decompress the composed messages, re-format the composed messages if desired
so that they will appear to have originated from the user's computer 262a, and re-route
the composed messages to message server 268 for delivery.
[0057] Certain properties or restrictions associated with messages that are to be sent from
and/or received by mobile device 100 can be defined (e.g. by an administrator in accordance
with IT policy) and enforced by message management server 272. These may include whether
mobile device 100 may receive encrypted and/or signed messages, minimum encryption
key sizes, whether outgoing messages must be encrypted and/or signed, and whether
copies of all secure messages sent from mobile device 100 are to be sent to a pre-defined
copy address, for example. Message management server 272 may also be adapted to provide
other control functions, such as only pushing certain message information or pre-defined
portions (e.g. "blocks") of a message stored on message server 268 to mobile device
100. For example, when a message is initially retrieved by mobile device 100 from
message server 268, message management server 272 is adapted to push only the first
part of a message to mobile device 100, with the part being of a pre-defined size
(e.g. 2 KB). The user can then request more of the message, to be delivered in similar-sized
blocks by message management server 272 to mobile device 100, possibly up to a maximum
pre-defined message size. Accordingly, message management server 272 facilitates better
control over the type of data and the amount of data that is communicated to mobile
device 100, and can help to minimize potential waste of bandwidth or other resources.
[0058] It will be understood by persons skilled in the art that message management server
272 need not be implemented on a separate physical server in LAN 250 or other network.
For example, some or all of the functions associated with message management server
272 may be integrated with message server 268, or some other server in LAN 250. Furthermore,
LAN 250 may comprise multiple message management servers 272, particularly in variant
implementations where a large number of mobile devices needs to be supported.
[0059] Having described the general mobile environment, the following description focuses
on a channel in a communications system for communication among mobile devices 100.
FIG. 5 is an illustration of such a channel 300 and a communications system, such
as that found in, for example, GSM systems using full rate (FR), adaptive multi-rate
(AMR), and other types of coding. The communications system generally includes coding
components 302 and decoding components 304 for coding and decoding, respectively,
a signal to be transmitted and received through the channel 300. In the context of
the mobile device 100, the coding components 302 and decoding components 304 are included
within, for example, DSP 160.
[0060] As shown in FIG. 5, a source signal 306, which is an information signal such as an
analog voice signal, that is to be transmitted, is provided to a source coder/quantizer
308, which quantizes and compresses the source signal 306 in order to reduce or remove
redundancies. The source coder 308 outputs a sequence of bits or, in some exemplary
embodiments, codewords, which are a tool used in communications to represent a combination
of bits that have been encoded for transmission. It will be understood that some distortion
of the signal may occur during the quantization stage due to lossy compression or
the like.
[0061] The source-coded signal is passed to a channel coder 310, which adds redundancy to
compensate for errors introduced in the channel during transmission. The channel coder
310 typically adds bits to the sequence to allow for error detection and correction,
for example, forward error checking (FEC) and cyclical redundancy check (CRC). The
output of the channel coder 310 is a series or sequence of bits. The signal may also
be otherwise encoded using various methods including, for example, time domain multiple
access (TDMA) signals, code domain multiple access (CDMA) signals, global system for
mobile communications (GSM) signals, or other types of communications signals.
[0062] It will be understood by one of skill in the art that the source coder 308 and the
channel coder 310 may be implemented in hardware or software or some combination thereof.
Further, either the source coder 308 or the channel coder 310 or the combination thereof
may be referred to as an encoder.
[0063] The channel-coded signal then passes through the channel 300
where it may encounter interference, noise or other situations that lead to corruption
of the bits that make up the signal.
[0064] The channel-coded signal is eventually received by a channel decoder 312 where the
redundancy in the channel-coded signal, such as the FEC and CRC information, is used
to check for or correct for errors in the signal and decode the channel-coded signal
to produce a coded signal.
[0065] The coded signal produced by the channel decoder 312 is passed to a lost frame handler
314, which then generates data to replace any lost frames in the received sequence.
The lost frame handler 314 uses Lost Frame Concealment (LFC) methods to replace a
lost frame, using information from previous frames and varying processing parameters
depending on certain conditions, to replace a lost frame or the like. These methods
are described in more detail with regards to FIGS. 6-8.
[0066] The coded signal is then passed to a source decoder/inverse quantizer 316 for decoding
to produce and output a recovered signal 318. In a codeword-based system, the source
decoder 316 will typically use a table look-up to map the received codeword to a parameter
value for output.
[0067] It will be understood by one of skill in the art that the channel decoder 312, lost
frame handler 314 and source decoder 316 may be implemented in hardware or software
or some combination thereof. Further, either the channel decoder 312 or the source
decoder 316 or the combination thereof including the lost frame handler 314 may be
referred to as a decoder.
[0068] It should also be understood by those skilled in the art that the components shown
in FIG. 5, provide one exemplary embodiment for source coding and decoding, channel
coding and decoding and that different processing schemes can be used in conjunction
with the lost frame handler 314.
[0069] For certain types of data, there are certain techniques that are used by the lost
frame handler 314 for handling lost data frames. The methods that are used employ
certain rules for dealing with lost data frames as well as subsequent data frames
that are received. Typically, a set of parameters is applied to a current data frame
based on the previous data frame. However, the processing that is typically applied
to the current data frame does not take into account channel conditions in certain
instances, which can have an effect on the quality of the recovered signal. The technique
can be applied to speech signals and in particular speech frames, on a frame or subframe
basis as is described in more detail below. The term data set used herein is meant
to cover a frame or a subframe of speech data.
[0070] Accordingly, the mobile device 100 employs a lost frame concealment method that takes
into account the channel conditions when processing a current speech frame while at
the same time taking into account whether the previous speech frame was a "good" frame,
i.e. the previous speech frame was received without error, or a "bad" frame, i.e.
the previous speech frame was received with an error. An exemplary embodiment of such
a lost frame concealment method 350 is shown in FIG. 6.
[0071] The lost frame concealment method 350 operates on speech frames that are received
and decoded by the channel decoder 312. The lost frame concealment method 350 begins
at step 352 at which point the Bad Frame Indicator (BFI) value of the current speech
frame is checked. If the BFI value is 1, indicating that the current speech frame
is bad, i.e. it has errors, then the lost frame concealment method 350 moves to step
354 at which point one or more parameters applied to the speech frame are limited
by a first set of values. If only one gain parameter is affected, then the first set
of values only includes one value. The lost frame concealment method 350 then ends
for the current speech frame. The lost frame concealment method 350 can begin once
more if another speech frame requires processing for lost frame handling.
[0072] Alternatively, if at step 352, the BFI value for the current speech frame is 0 indicating
that the current speech frame is good, i.e. it has no errors, then the lost frame
concealment method 350 proceeds to step 356 at which point it is determined whether
the BFI value for the previous speech frame was 1. If this is false, then both the
current and previous data frames are good (i.e. no errors), and the lost frame concealment
method 350 moves to step 358 at which point normal source decoding is employed by
the source decoder 316. However, if the BFI value for the previous speech frame was
1, then the lost frame concealment method 350 moves to step 360 at which point the
quality of the channel 300 is determined by checking the value of a Channel Quality
Indicator (CQI). If the CQI indicates good channel conditions, then the lost frame
concealment method 350 proceeds to step 358 at which point normal processing is applied
to the speech frame. Otherwise, the lost frame error concealment method 350 proceeds
to step 362 at which point the speech frame is processed using a second set of values
that may be different than the first set of values. For instance, one or more parameters
that are applied to the speech frame, can be limited according to the corresponding
one or more values in the second set of values. The amount of limitation applied in
steps 354 and 362 can be different.
[0073] The CQI can be represented by various parameters including Bit Error Rate (BER),
BLock Error Ratio (BLER), Signal to Noise Ratio (SNR), as well as other suitable known
parameters, which correspond to different measurements that indicate channel condition.
Alternatively, the CQI can be a specially defined parameter as long as it indicates
channel conditions. In any of these cases, the CQI is compared with a threshold value
to determine whether the channel 300 is good. For instance, if the CQI is BER, then
the BER can be compared to a threshold at step 360 and if the BER is greater than
or equal to the threshold, then the current conditions for the channel 300 are poor
and the method 350 moves to step 362. Otherwise if the BER is less than the threshold,
then the current conditions for the channel is good and the method moves to step 358.
[0074] A value for the threshold can be obtained, for the channel quality indicator that
is used, through a priori knowledge of the channel and its effect on the channel quality
indicator under good and bad channel conditions. Alternatively, this information can
be obtained through testing to obtain suitable values for the threshold.
[0075] With regards to speech traffic channels, techniques such as Adaptive Multi-Rate (AMR)
speech codec error concealment of lost frames, and substitution and muting of lost
frames for Enhanced Full Rate (EFR) speech traffic channels, have been typically used
to process speech frames depending on whether errors are detected in current and previous
speech frames. For instance, when no error is detected in a current speech frame but
the previous speech frame had an error, these techniques conventionally always apply
a change to the gains applied to the current speech frame. However, this processing
approach is not appropriate under all circumstances.
[0076] For instance, with respect to 3GPP TS 46.061 substitution and muting of lost frames
for Enhanced Full Rate (EFR) speech traffic channels, in previous solutions for substitution
and muting of lost speech frames, when no error was detected in the received speech
frame but the previous received speech frame was bad, the Long Term Prediction (LTP)
gain and fixed codebook gain was limited below the values used for the last received
good frame. This approach may provide acceptable performance when the channel condition
is poor and the probability of the current speech frame being good (i.e. no errors)
is low.
However, this approach will degrade speech performance greatly when the channel condition
is actually good and the previous frame is bad due to various reasons such as when
a Fast Associated Control CHannel (FACCH) frame is used, for example. An FACCH channel
is inserted based on the current need of the communication system. When a speech frame
is replaced by an FACCH frame, the BFI value is set to "bad" because the frame contains
no useful information for speech decoding.
[0077] This can be further understood by looking at a situation involving handover under
good channel conditions. In this case, the wireless network 200 will send out a series
of FACCH frames until it receives a response from the mobile device 100. Analysis
of network activity shows that a typical pattern of frames in this instance consists
of a dozen frames with FACCH frames embedded in every other frame. When the current
frame is bad (i.e. BFI = 1) and the BFI value for the previous frame was good or bad
(prevBFl = 0 or 1), the LTP gain and fixed codebook gain are replaced by attenuated
versions of the values of LTP gain and fixed codebook gain used from one or more previous
frames. However, when the current frame is good (i.e. BFI = 0) and the previous frame
was bad (i.e. prevBFI = 1), the LTP gain and fixed codebook gain are again replaced
by attenuated values from one or more previous frames. If frames are received such
that there is an alternating pattern of frames with (BFI=1, prevBFI = 0/1) and (BFI=0,
prevBFI =1), then despite the fact that good speech frames are received half of the
time, the result of the conventional methods used in AMR and EFR speech codec error
concealment unit is that there is attenuated speech for the first 6 frames and muting
for the rest of the frames. It should be noted that the length of each speech frame
is 20 ms, and this result of attenuated and muted speech will leave a noticeable gap
in the speech in the recovered signal 318.
[0078] Another approach to better handle this situation is to apply the lost frame concealment
method 350. In this case, when the current frame is good and the previous frame was
bad (BFI=0, prevBFI =1), the channel quality indicator is checked first. The LTP-gain
and fixed codebook gain will be limited when the channel quality indicator indicates
a poor channel condition. In this way, good frames that are in between bad speech
frames will be used and the result is an improvement in speech quality. Accordingly,
in this example, the lost frame concealment method 350 makes use of the distinction
between frame erasures due to FACCH versus poor channel conditions. This will now
be described in more detail with regards to an exemplary embodiment.
[0079] The lost frame concealment method 350 can be part of a modified frame substitution
and muting procedure, which can be used by an AMR speech codec receiving end when
one or more lost speech frames are received. In this case, the purpose of error concealment
is to conceal the effect of lost AMR speech frames. The purpose of muting the received
speech frames in the case of several lost frames is to indicate the breakdown of the
channel to the user and to avoid generating possibly annoying sounds as a result from
the error concealment procedure.
[0080] For the purposes of error detection, if the most sensitive bits of AMR speech data
are received in error, the wireless network 200 can set a flag RX_TYPE to SPEECH_BAD
in which case the BFI flag is set to 1 to indicate a bad data frame. If an SID frame
is received in error, the wireless network 200 can set the RX_TYPE flag to SID_BAD
in which case the BFI flag is also set to 1 to indicate a bad data frame. If these
flags are set, the decoder components 304 shall perform parameter substitution to
conceal errors. The RX_TYPE flag can be set to SPEECH_PROBABLY_DEGRADED by using channel
quality information from the channel decoder 312, in which case the Potentially Degraded
Frame Indication (PDFI) flag is also set.
[0081] In the case of lost speech frames, normal decoding of such frames would result in
very unpleasant noise effects. In order to improve subjective quality, lost speech
frames are typically substituted with either a repetition or an extrapolation of at
least one previous good speech frame. This substitution is done so that it gradually
will decrease the output level, resulting in silence at the output recovered signal
318, if several consecutive lost speech frames are received.
[0082] An exemplary solution for substitution and muting incorporates a state machine with
seven states as shown in FIG. 7. The state machine starts in state 0. Each time a
bad frame is detected, the state counter is incremented by one and is saturated when
it reaches 6. Each time a good speech frame is detected, the state counter is reset
to zero, except when in state 6, at which point the state counter is set to 5. The
value of the state counter indicates the quality of the channel: the larger the value
of the state counter, the worse the channel quality is. In addition to this state
machine, the BFI value for the previously received data frame is checked (i.e. prevBFI).
The processing generally depends on the value of the state variable. However, in states
0 and 5, the processing also depends on the two flags BFI and prevBFI, as will now
be explained.
[0083] When BFI = 0, prevBFI = 0, and state = 0, there is no error that is detected in the
currently received or in the previously received speech frame. In this context no
error means that, there is no error detected for a system like 802.11 or no error
in CRC protected fields in GSM. That is, the most sensitive bits are received with
no error but the less sensitive bits may contain some errors but do not have a significant
effect on speech decoding. The received speech parameters are used in the normal way
during speech synthesis. The speech parameters for the current frame are saved. These
actions correspond to step 358 for method 350.
[0084] When BFI = 0, prevBFI = 1, and the state = 0 or 5, no error is detected in the currently
received speech frame, but the previously received speech frame was bad. The channel
conditions are checked using a channel quality indicator as in step 360 of method
350. If the channel conditions are good, the LTP gain and fixed codebook gain are
not limited and normal decoding takes place using the received parameters, which corresponds
to step 358 of method 350. However, if the channel conditions are poor, then the LTP
gain and fixed codebook gain are then limited below the values used for the last subframe
in the last received good frame as shown in equations 1 and 2 respectively. This corresponds
to step 362 in method 302. A subframe can have a time interval on the order of milliseconds
such as 5 ms for example and there are several subframes in a frame. For example,
there can be four subframes in a frame. BFI and prevBFI are only updated on a frame-by-frame
basis.
In equation 1,
gP is the current decoded LTP gain that is applied to the current frame,
gp(-1) is the LTP gain that was used for the last subframe in the last good frame (i.e.
when BFI was 0). In equation 2,
gc is the current decoded fixed codebook gain that is applied to the current frame and
gc(-1) is the fixed codebook gain used for the last subframe in the last good frame
(i.e. when BFI was 0). The rest of the received speech parameters are used normally
during speech synthesis. The speech parameters for the current frame are saved. This
operation corresponds to step 362 in method 302.
[0085] It is understood that a fixed codebook contains excitation vectors for speech synthesis
filters. The contents of the codebook are non-adaptive (i.e. fixed). In an adaptive
multi-rate codec, the fixed codebook is implemented using an algebraic codebook. Alternatively,
an adaptive codebook contains excitation vectors that are adapted for every subframe.
The adaptive codebook is derived from a long-term filter state. The lag value can
be viewed as an index into the adaptive codebook.
[0086] When BFI = 1, prevBFI = 0 or 1, and the state = 1...6, an error is detected in the
currently received speech frame and a substitution and muting procedure is started.
The LTP gain and fixed codebook gain are replaced by attenuated values from several
previous subframes according to equations 3 and 4. This corresponds to step 354 in
method 350.
In equation 3,
gp is the current decoded LTP gain,
gp (-1), ...,
gp (-
n) are the LTP gains used for the last n subframes,
median5() is a 5-point median operation,
P(state) is an attenuation factor:
(P(1) = 0.98,
P(2) = 0.98,
P(3) = 0.8,
P(4) = 0.3,
P(5) = 0.2,
P(6) = 0.2), and
state is the state value. In equation 4,
gc is the current decoded fixed codebook gain,
gc (-1), ...,
gc(-
n) are the fixed codebook gains used for the last n subframes,
median5() is a 5-point median operation,
C(state) is an attenuation factor: (
C(1) = 0.98,
C(2) = 0.98, C(3) = 0.98,
C(4) = 0.98,
C(5) = 0.98,
C(6) = 0.7),
state is the state value, and n is a positive integer.
[0087] The higher the state value is, the more the gains are attenuated. Also the memory
of the predictive fixed codebook gain is updated by using the average value of the
past four values in the memory as shown in equation 5, and the past LSFs are shifted
towards their mean as shown in equation 6.
In equation 6, α = 0.95,
lsf_q1 and
lsf_q2 are two sets of LSF-vectors for the current frame,
past_lsf_q is
lsf_q2 from the previous frame, and
mean_lsf is the average LSF-vector.
[0088] The LTP-lag values can be replaced by the past value from the 4
th subframe of the previous frame or slightly modified values based on the last correctly
received value. The received fixed codebook innovation pulses from the erroneous frame
can be used in the state in which they were received when corrupted data is received.
In the case where no data was received, random fixed codebook indices can be employed.
[0089] Referring now to FIG. 8, shown therein is a flow chart diagram of another exemplary
embodiment of a lost frame concealment method 400. The method 400 is somewhat similar
to the method 350. The method begins at step 402 at which point it is determined whether
the current data frame is erroneous or bad. If this is true then the method 400 moves
to step 404 at which point the data frame is processed using one or more parameters
and a first set of values is used to limit one or more of the parameters. If the current
data frame is not erroneous then the method 400 moves to step 406 in which the parameters
are used without modification or limitation to process the current data frame. Accordingly,
the method 400 provides the same benefit as the method 350 when the condition of the
channel 300 is good but the method 400 is not as robust as the method 350 when the
condition of the channel 300 is poor. In the context of the example that was just
given, when BFI = 0, prevBFI = 0, and state = 0 or when BFI = 0, prevBFI = 1, and
state = 0 or 5, then no error is detected in the received speech frame but an error
may or may not have been detected in the previous received speech frame. Accordingly,
the received speech parameters are used in the normal way during speech synthesis
for the current received speech frame, on a frame or a subframe basis, and the speech
parameters are saved for the current frame.
[0090] The error concealment handling embodiments described herein are intended to provide
improved voice quality for the mobile device 100 (e.g. a GSM handset) under both good
and poor channel conditions. It will be further understood that the system and method
of coding and decoding signals and handling lost frames described above may be implemented
as either hardware or software or some combination thereof. Further, methods and software
may be implemented as executable software instructions stored on computer-readable
media, which may include transmission-type media, which may be executed in a computer.
[0091] It should be understood that various modifications can be made to the embodiments
described and illustrated herein, without departing from the embodiments, the general
scope of which is defined in the appended claims.
1. A lost frame concealment method (350, 400) for processing data frames received from
transmission over a communications channel (300), wherein the method (350, 400) comprises:
determining (352, 402) whether a current data frame is a bad frame or a good frame,
a data frame being a bad frame when determined to be received with error or used for
control purposes and the data frame being a good frame when determined to be received
without error and not used for control purposes; and
performing (354) source decoding on the current data frame with one or more parameters
wherein:
if the current data frame is a bad frame the one or more parameters are limited by
a first set of one or more values; and
if the current data frame is a good frame and a previous data frame is a bad frame
a condition of the communications channel is checked to determine whether to limit
the one or more parameters.
2. The method of claim 1, wherein the method comprises the step of performing (358) source
decoding on the current data frame with one or more parameters, wherein the one or
more parameters are not limited when the current and previous data frames are good
frames.
3. The method of claim 1, wherein if the current data frame is a good frame and the previous
data frame is a bad data frame, the method further comprises:
determining a value for a channel quality indicator to determine the condition of
the communications channel (300) by comparing (360) the value of the channel quality
indicator to a threshold;
performing (358) the step of source decoding on the current data frame with one or
more parameters, wherein the one or more parameters are not limited if the condition
of the communications channel (300) is good; and
performing (362) the step of source decoding on the current data frame with one or
more parameters, the one or more parameters being limited by a second set of one or
more values if the condition of the communications channel (300) is bad.
4. The method of claim 3, wherein the second set of one or more values is different from
the first set of one or more values.
5. The method of claim 3 or claim 4, wherein the channel quality indicator is one of
a Bit Error Rate, BER, a BLock Error Ratio, BLER, a Signal to Noise Ratio 'SNR' and
a specially defined parameter that indicates the condition of the communication channel
(300).
6. The method of any one of claim 1 to 5, wherein the data frames comprise speech frames,
and the method is applied to Adaptive Multi-Rate, AMR speech decoding for concealing
the effect of lost AMR speech frames.
7. The method of claim 6, wherein a state machine is used to indicate the condition of
the communications channel (300), and the method further comprises:
starting the state machine in state 0;
incrementing a state counter to enter a subsequent numbered state each time a bad
frame is detected, the incrementing being limited to 6; and
resetting the state counter to zero each time a good speech frame is detected except
when in state 6 in which case the state counter is set to 5.
8. The method of claim 7, wherein performing (358, 406) the step of source decoding on
the current data frame with one or more parameters, wherein the one or more parameters
are not limited is performed in state 0, the method comprises not limiting Long Term
Prediction, LTP gain and fixed codebook gain, performing normal source decoding and
saving the current frame of speech parameters.
9. The method of claim 7 or claim 8, wherein the steps of claim 3 are performed in state
0 or state 5 when the current data frame is a good data frame and the previous data
frame is a bad data frame, and wherein the step of performing (362) source decoding
on the current data frame with one or more parameters, the one or more parameters
being limited by the second set of one or more values comprises limiting LTP gain
and fixed codebook gain below values used for the last subframe in the last received
good speech frame according to:
where
gp is a current LTP gain that is applied to the current speech frame,
gp(-1) is the LTP gain that was used for the last subframe in the last good received
speech frame,
gc is a current decoded fixed codebook gain that is applied to the current speech frame
and
gc(-1) is a fixed codebook gain that was used for the last subframe of the last good
received speech frame, and the method further comprises using any remaining received
speech parameters normally, and saving the speech parameters for the current speech
frame.
10. The method of claim 7 or claim 8, wherein the step of performing (354, 404) source
decoding on the current data frame with one or more parameters the one or more parameters
being limited by the first set of one or more values is performed in all states when
the current data frame is a bad data frame, and said step comprises limiting LTP gain
and fixed codebook gain below values used for the last subframe in the last received
good speech frame according to:
and
where
gp is a current decoded LTP gain,
gp(-1),...,
gp(-
n) are LTP gains used for the last n subframes,
median5() is a 5-point median operation,
P(state) is
an attenuation factor defined by: (
P(1) = 0.98,
P(2) = 0.98,
P(3) = 0.8,
P(4) = 0.3,
P(5) = 0.2,
P(6) = 0.2),
gc is a current decoded fixed codebook gain,
gc(-1),...,
gc(-
n) are fixed codebook gains used for the last n subframes,
C(state) is an attenuation factor defined by: (
C(1) = 0.98,
C(2) = 0.98,
C(3) = 0.98,
C(4) = 0.98,
C(5) = 0.98,
C(6) = 0.7),
state is the state value, and n is a positive integer.
11. A computer program product comprising a computer readable medium embodying program
code means executable by a processor (102) of a communications device (100) for causing
said communications device (100) to implement the steps of the lost frame concealment
method (350, 400) of any one of claims 1 to 10.
12. A communications device (100) comprising:
a microprocessor (102) configured to control the operation of the communications device
(100);
a communication subsystem (104) connected to the microprocessor (102), the communication
subsystem (104) being configured to send and receive wireless data over a communications
channel (300);
a channel decoder (312) configured to decode data frames received over the communications
channel (300); and
a lost frame handler (314) configured to process the received data frames for lost
frames, the lost frame handler (314) being configured to perform the steps of the
method (350, 400) of any one of claims 1 to 10.
13. A communication system (302, 304) for coding and decoding an information signal sent
through a communications channel (300), wherein the system (302, 304) comprises:
an encoder (302) configured to encode the information signal and send the encoded
information signal over the communications channel (300); and
a decoder (304) configured to receive and decode the encoded information signal to
produce a recovered signal, wherein the decoder is configured to perform the steps
of the method (350, 400) of any one of claims 1 to 10.
1. Rahmenverlustmaskierungsverfahren (350, 400) zum Verarbeiten von von einer Übertragung
über einen Kommunikationskanal (300) empfangenen Datenrahmen, wobei das Verfahren
(350, 400) umfasst:
Ermitteln (352, 402), ob ein aktueller Datenrahmen ein unzureichender Rahmen oder
ein zureichender Rahmen ist, wobei ein Datenrahmen ein unzureichender Rahmen ist,
wenn ermittelt wird, dass er fehlerhaft empfangen oder für Kontrollzwecke verwendet
wird, und der Datenrahmen ein zureichender Rahmen ist, wenn ermittelt wird, dass er
fehlerlos empfangen und nicht für Kontrollzwecke verwendet wird; und
Durchführen (354) einer Quellendecodierung in dem aktuellen Datenrahmen mit einem
oder mehreren Parametern; wobei:
wenn der aktuelle Datenrahmen ein unzureichender Rahmen ist, der eine oder die mehreren
Parameter durch eine erste Menge eines oder mehrerer Werte begrenzt sind; und
wenn der aktuelle Datenrahmen ein zureichender Rahmen und ein vorheriger Datenrahmen
ein unzureichender Rahmen ist, eine Beschaffenheit des Kommunikationskanals überprüft
wird, um zu ermitteln, ob der eine oder die mehreren Parameter zu begrenzen sind.
2. Verfahren gemäß Anspruch 1, wobei das Verfahren den Schritt des Durchführens (358)
einer Quellendecodierung in dem aktuellen Datenrahmen mit einem oder mehreren Parametern
umfasst, wobei der eine oder die mehreren Parameter nicht begrenzt sind, wenn der
aktuelle und der vorherige Datenrahmen zureichende Rahmen sind.
3. Verfahren gemäß Anspruch 1, wobei, wenn der aktuelle Datenrahmen ein zureichender
Rahmen und der vorherige Datenrahmen ein unzureichender Datenrahmen ist, das Verfahren
ferner umfasst:
Ermitteln eines Werts für einen Kanalqualitätsindikator, um die Beschaffenheit des
Kommunikationskanals (300) zu ermitteln, indem der Wert des Kanalqualitätsindikators
mit einem Schwellwert verglichen (360) wird;
Durchführen (358) des Schritts der Quellendecodierung in dem aktuellen Datenrahmen
mit einem oder mehreren Parametern, wobei der eine oder die mehreren Parameter nicht
begrenzt sind, wenn die Beschaffenheit des Kommunikationskanals (300) zureichend ist;
und
Durchführen (362) des Schritts der Quellendecodierung in dem aktuellen Datenrahmen
mit einem oder mehreren Parametern, wobei der eine oder die mehreren Parameter durch
eine zweite Menge eines oder mehrerer Werte begrenzt sind, wenn die Beschaffenheit
des Kommunikationskanals (300) unzureichend ist.
4. Verfahren gemäß Anspruch 3, wobei sich die zweite Menge eines oder mehrerer Werte
von der ersten Menge eines oder mehrerer Werte unterscheidet.
5. Verfahren gemäß Anspruch 3 oder Anspruch 4, wobei der Kanalqualitätsindikator eine
Bitfehlerrate (Bit Error Rate, ,BER'), eine Blockfehlerrate (Block Error Ratio, 'BLER'),
ein Signal-Rausch-Verhältnis (Signal to Noise Ratio, ,SNR') oder ein eigens definierter
Parameter ist, der die Beschaffenheit des Kommunikationskanals (300) indiziert.
6. Verfahren gemäß einem der Ansprüche 1 bis 5, wobei die Datenrahmen Sprachrahmen umfassen
und das Verfahren auf eine Adaptive-Multi-Rate-(AMR-) Sprachdecodierung zum Maskieren
der Wirkung von AMR-Sprachrahmenverlusten angewendet wird.
7. Verfahren gemäß Anspruch 6, wobei eine Zustandsmaschine verwendet wird, um die Beschaffenheit
des Kommunikationskanals (300) zu indizieren, und das Verfahren ferner umfasst:
Starten der Zustandsmaschine im Zustand 0;
Hochzählen eines Zustandszählers, um einen nachfolgenden nummerierten Zustand jedes
Mal einzugeben, wenn ein unzureichender Rahmen erkannt wird, wobei das Hochzählen
auf 6 begrenzt ist; und
Rücksetzen des Zustandszählers auf null jedes Mal, wenn ein zureichender Sprachrahmen
erkannt wird, außer im Zustand 6, in welchem Fall der Zustandszähler auf 5 gesetzt
wird.
8. Verfahren gemäß Anspruch 7, wobei das Durchführen (358, 406) des Schritts der Quellendecodierung
in dem aktuellen Datenrahmen mit einem oder mehreren Parametern, wobei der eine oder
die mehreren Parameter nicht begrenzt sind, im Zustand 0 durchgeführt wird, das Verfahren
das Nichtbegrenzen des Gewinns einer Langzeitvorhersage (Long Term Prediction, 'LTP')
und des Gewinns eines festen Codebuchs, das Durchführen einer normalen Quellendecodierung
und das Speichern des aktuellen Sprachparameterrahmens umfasst.
9. Verfahren gemäß Anspruch 7 oder Anspruch 8, wobei die Schritte von Anspruch 3 im Zustand
0 oder Zustand 5 durchgeführt werden, wenn der aktuelle Datenrahmen ein zureichender
Datenrahmen und der vorherige Datenrahmen ein unzureichender Datenrahmen ist, und
wobei der Schritt des Durchführens (362) einer Quellendecodierung in dem aktuellen
Datenrahmen mit einem oder mehreren Parametern, wobei der eine oder die mehreren Parameter
durch die zweite Menge eines oder mehrerer Werte begrenzt sind, das Begrenzen von
LTP-Gewinn und Gewinn eines festen Codebuchs unter für den letzten Unterrahmen im
letzten empfangenen zureichenden Sprachrahmen verwendeten Werten umfasst gemäß:
wenn
gp ein aktueller LTP-Gewinn ist, der auf den aktuellen Sprachrahmen angewendet wird,
gp(-1) der LTP-Gewinn ist, der für den letzten Unterrahmen im letzten zureichenden empfangenen
Sprachrahmen verwendet wurde,
gc ein aktueller decodierter Gewinn eines festen Codebuchs ist, der auf den aktuellen
Sprachrahmen angewendet wird, und
gc(-1) ein Gewinn eines festen Codebuchs ist, der für den letzten Unterrahmen des letzten
zureichenden empfangenen Sprachrahmens verwendet wurde, und das Verfahren ferner ein
normales Verwenden von beliebigen übrigen empfangenen Sprachparametern und ein Speichern
der Sprachparameter für den aktuellen Sprachrahmen umfasst.
10. Verfahren gemäß Anspruch 7 oder Anspruch 8, wobei der Schritt des Durchführens (354,
404) einer Quellendecodierung in dem aktuellen Datenrahmen mit einem oder mehreren
Parametern, wobei der eine oder die mehreren Parameter durch die erste Menge eines
oder mehrerer Werte begrenzt sind, in allen Zuständen durchgeführt wird, wenn der
aktuelle Datenrahmen ein unzureichender Datenrahmen ist, und der Schritt das Begrenzen
von LTP-Gewinn und Gewinn eines festen Codebuchs unter für den letzten Unterrahmen
im letzten empfangenen zureichenden Sprachrahmen verwendeten Werten umfasst gemäß:
und
wenn
gp ein aktueller decodierter LTP-Gewinn ist,
gp(-1),...,
gp(
-n) für die letzten n Unterrahmen verwendete LTP-Gewinne sind,
Median5() eine 5-Punkt-Medianoperation ist,
P(Zustand) ein Abschwächungsfaktor ist, definiert durch: (
P(1) = 0,98,
P(2) = 0,98,
P(3) = 0,8,
P(4) = 0,3,
P(5) = 0,2,
P(6) = 0,2),
gc ein aktueller decodierter Gewinn eines festen Codebuchs ist,
gc(-1),...,
gc(-
n) für die letzten n Unterrahmen verwendete Gewinne eines festen Codebuchs sind,
C(Zustand) ein Abschwächungsfaktor ist, definiert durch: (
C(1) = 0,98,
C(2) = 0,98,
C(3) = 0,98,
C(4) = 0,98,
C(5) = 0,98,
C(6) = 0,7),
Zustand der Zustandswert ist und n eine positive ganze Zahl ist.
11. Computerprogrammprodukt, das ein computerlesbares Medium umfasst, das von einem Prozessor
(102) einer Kommunikationsvorrichtung (100) ausführbare Programmcodemittel darstellt,
um zu bewirken, dass die Kommunikationsvorrichtung (100) die Schritte des Rahmenverlustmaskierungsverfahrens
(350, 400) gemäß einem der Ansprüche 1 bis 10 durchführt.
12. Kommunikationsvorrichtung (100), die umfasst:
einen Mikroprozessor (102), der konfiguriert ist, um den Betrieb der Kommunikationsvorrichtung
(100) zu kontrollieren;
ein mit dem Mikroprozessor (102) verbundenes Kommunikationssubsystem (104), wobei
das Kommunikationssubsystem (104) konfiguriert ist, um drahtlose Daten über einen
Kommunikationskanal (300) zu versenden und zu empfangen;
einen Kanaldecodierer (312), der konfiguriert ist, um über den Kommunikationskanal
(300) empfangene Datenrahmen zu decodieren; und
ein Rahmenverluststeuerprogramm (314), das konfiguriert ist, um die empfangenen Datenrahmen
nach Rahmenverlusten durchzuarbeiten, wobei das Rahmenverluststeuerprogramm (314)
konfiguriert ist, um die Schritte des Verfahrens (350, 400) gemäß einem der Ansprüche
1 bis 10 durchzuführen.
13. Kommunikationssystem (302, 304) zum Codieren und Decodieren eines durch einen Kommunikationskanal
(300) gesendeten Informationssignals, wobei das System (302, 304) umfasst:
einen Codierer (302), der konfiguriert ist, um das Informationssignal zu codieren
und das codierte Informationssignal über den Kommunikationskanal (300) zu versenden;
und
einen Decodierer (304), der konfiguriert ist, um das codierte Informationssignal zu
empfangen und decodieren, um ein wiederhergestelltes Signal zu erzeugen, wobei der
Decodierer konfiguriert ist, um die Schritte des Verfahrens (350, 400) gemäß einem
der Ansprüche 1 bis 10 durchzuführen.
1. Un procédé de masquage de trames perdues (350, 400) pour traiter des trames de données
reçues d'une transmission sur un canal de communication (300), où le procédé (350,
400) comporte les étapes visant à :
déterminer (352, 402) si une trame de données actuelle est une mauvaise trame ou une
bonne trame, une trame de données étant une mauvaise trame lorsqu'il est déterminé
qu'elle est reçue avec une erreur ou utilisée à des fins de contrôle et la trame de
données étant une bonne trame lorsqu'il est déterminé qu'elle est reçue sans erreur
et n'est pas utilisée à des fins de contrôle ; et
réaliser (354) un décodage de source sur la trame de données actuelle avec un ou plusieurs
paramètre(s) ; où :
si la trame de données actuelle est une mauvaise trame l'un ou les plusieurs paramètre(s)
est/sont limité(s) par un premier ensemble d'une ou de plusieurs valeur(s) ; et
si la trame de données actuelle est une bonne trame et qu'une trame de données précédente
est une mauvaise trame une condition du canal de communication est vérifiée pour déterminer
s'il faut limiter l'un ou les plusieurs paramètre(s).
2. Le procédé de la revendication 1, où le procédé comporte l'étape visant à réaliser
(358) un décodage de source sur la trame de données actuelle avec un ou plusieurs
paramètre(s), où l'un ou les plusieurs paramètre(s) n'est/ne sont pas limité(s) lorsque
les trames de données actuelle et précédente sont de bonnes trames.
3. Le procédé de la revendication 1, où si la trame de données actuelle est une bonne
trame et la trame de données précédente est une mauvaise trame de données, le procédé
comporte de plus les étapes visant à :
déterminer une valeur pour un indicateur de qualité de canal pour déterminer la condition
du canal de communication (300) en comparant (360) la valeur de l'indicateur de qualité
de canal avec un seuil ;
réaliser (358) l'étape de décodage de source sur la trame de données actuelle avec
un ou plusieurs paramètre(s), où l'un ou les plusieurs paramètre(s) n'est/ne sont
pas limité(s) si la condition du canal de communication (300) est bonne ; et
réaliser (362) l'étape de décodage de source sur la trame de données actuelle avec
un ou plusieurs paramètre(s), l'un ou les plusieurs paramètre(s) étant limité(s) par
un deuxième ensemble d'une ou de plusieurs valeur(s) si la condition du canal de communication
(300) est mauvaise.
4. Le procédé de la revendication 3, où le deuxième ensemble d'une ou de plusieurs valeur(s)
est différent du premier ensemble d'une ou de plusieurs valeur(s).
5. Le procédé de la revendication 3 ou de la revendication 4, où l'indicateur de qualité
de canal est un indicateur parmi un taux d'erreurs sur les bits (BER), un taux d'erreurs
sur les blocs (BLER), un rapport signal-bruit (SNR) et un paramètre spécialement défini
qui indique la condition du canal de communication (300).
6. Le procédé de n'importe laquelle des revendications 1 à 5, où les trames de données
comportent des trames vocales, et le procédé est appliqué à un décodage de parole
AMR (« Adaptive Multi-Rate ») pour masquer l'effet de trames vocales AMR perdues.
7. Le procédé de la revendication 6, où une machine à états est utilisée pour indiquer
la condition du canal de communication (300), et le procédé comporte de plus les étapes
visant à :
démarrer la machine à états dans l'état 0 ;
incrémenter un compteur d'états pour entrer un état numéroté subséquent chaque fois
qu'une mauvaise trame est détectée, l'incrémentation étant limitée à 6 ; et
remettre le compteur d'états à zéro chaque fois qu'une bonne trame vocale est détectée
sauf dans l'état 6, auquel cas le compteur d'états est réglé à 5.
8. Le procédé de la revendication 7, où la réalisation (358, 406) de l'étape de décodage
de source sur la trame de données actuelle avec un ou plusieurs paramètre(s) où l'un
ou les plusieurs paramètre(s) n'est/ne sont pas limité(s) est réalisée dans l'état
0, le procédé comporte la non limitation du gain de prédiction à long terme (LTP)
et du gain de répertoire fixe, la réalisation d'un décodage de source normal et la
sauvegarde de la trame actuelle de paramètres vocaux.
9. Le procédé de la revendication 7 ou de la revendication 8, où les étapes de la revendication
3 sont réalisées dans l'état 0 ou l'état 5 lorsque la trame de données actuelle est
une bonne trame de données et la trame de données précédente est une mauvaise trame
de données, et où l'étape de réalisation (362) d'un décodage de source sur la trame
de données actuelle avec un ou plusieurs paramètre(s), l'un ou les plusieurs paramètre(s)
étant limité(s) par le deuxième ensemble d'une ou de plusieurs valeur(s), comporte
la limitation du gain LTP et du gain de répertoire fixe sous les valeurs utilisées
pour la dernière sous-trame dans la dernière bonne trame vocale reçue selon :
où
gP est un gain LTP actuel
qui est appliqué à la trame vocale actuelle,
gP(-1) est le gain LTP qui a été utilisé pour la dernière sous-trame dans la dernière
bonne trame vocale reçue,
gC est un gain de répertoire fixe décodé actuel qui est appliqué à la trame vocale actuelle
et
gC(-1) est un gain de répertoire fixe qui a été utilisé pour la dernière sous-trame
de la dernière bonne trame vocale reçue, et le procédé comporte de plus l'utilisation
de n'importe quels paramètres vocaux reçus restants normalement, et la sauvegarde
des paramètres vocaux pour la trame vocale actuelle.
10. Le procédé de la revendication 7 ou de la revendication 8, où l'étape de réalisation
(354, 404) d'un décodage de source sur la trame de données actuelle avec un ou plusieurs
paramètre(s), l'un ou les plusieurs paramètre(s) étant limité(s) par le premier ensemble
d'une ou de plusieurs valeur(s), est réalisée dans tous les états lorsque la trame
de données actuelle est une mauvaise trame de données, et ladite étape comporte la
limitation du gain LTP et du gain de répertoire fixe sous les valeurs utilisées pour
la dernière sous-trame dans la dernière bonne trame vocale reçue selon :
et
où
gP est un gain LTP décodé actuel,
gP(-1),...,
gP(-
n) sont des gains LTP utilisés pour les n dernières sous-trames,
médiane5() est une opération médiane à 5 points,
P(état) est un facteur d'atténuation défini par : (
P(1) = 0,98,
P(2) = 0,98,
P(3) = 0,8,
P(4) = 0,3,
P(5) = 0,2,
P(6) = 0,2),
gC est un gain de répertoire fixe décodé actuel,
gC(-1),...,
gC(-
n) sont des gains de répertoire fixe utilisés pour les n dernières sous-trames,
C(état) est un facteur d'atténuation défini par : (
C(1) = 0,98,
C(2) = 0,98,
C(3) = 0,98,
C(4) = 0,98,
C(5) = 0,98,
C(6) = 0,7),
état est la valeur d'état, et n est un entier positif.
11. Un produit de programme informatique comportant un support lisible par ordinateur
intégrant un moyen de code de programme exécutable par un processeur (102) d'un dispositif
de communication (100) pour amener ledit dispositif de communication (100) à implémenter
les étapes du procédé de masquage de trames perdues (350, 400) de n'importe laquelle
des revendications 1 à 10.
12. Un dispositif de communication (100) comportant :
un microprocesseur (102) configuré pour contrôler le fonctionnement du dispositif
de communication (100) ;
un sous-système de communication (104) connecté au microprocesseur (102), le sous-système
de communication (104) étant configuré pour envoyer et recevoir des données sans fil
sur un canal de communication (300) ;
un décodeur de canal (312) configuré pour décoder des trames de données reçues sur
le canal de communication (300) ; et
un gestionnaire de trames perdues (314) configuré pour traiter les trames de données
reçues pour les trames perdues, le gestionnaire de trames perdues (314) étant configuré
pour réaliser les étapes du procédé (350, 400) de n'importe laquelle des revendications
1 à 10.
13. Un système de communication (302, 304) pour coder et décoder un signal d'information
envoyé par le biais d'un canal de communication (300), où le système (302, 304) comporte
:
un codeur (302) configuré pour coder le signal d'information et envoyer le signal
d'information codé sur le canal de communication (300) ; et
un décodeur (304) configuré pour recevoir et décoder le signal d'information codé
afin de produire un signal rétabli, où le décodeur est configuré pour réaliser les
étapes du procédé (350, 400) de n'importe laquelle des revendications 1 à 10.